It is well known to use synchronous motors with permanent magnet rotors in a traction drive for an elevator. In such motors, a stator (or armature) comprises one or more windings for generating a rotating magnetic field around a motor axis. This magnetic field interacts with the magnetic field produced by one or more permanent magnets provided on a rotor that is accommodated inside the stator bore, thereby rotating the rotor.
U.S. Pat. No. 6,822,359 B2 shows a rotor for a synchronous motor wherein the permanent magnets are arranged on the rotor's circumferential surface. However, these permanent magnets must have a high flux density to generate the necessary torques to lift an elevator's car. Such permanent magnets with high flux density, made from sintered Rare Earth materials for example, are expensive, thereby increasing the overall costs for the traction drive. Moreover, if the flux density of such external permanent magnets is too small, these magnets cannot resist current overloads within the stator's windings without loosing their magnetic characteristic.
Thus it is known, for example from U.S. Pat. No. 5,697,848 or U.S. Patent Publication No. 2005/0168089 A1 to place the permanent magnets within the rotor itself. Thereby, rotor material adjacent to the permanent magnets increases the magnetic flux in radial direction, allowing use of magnets with lower flux density that furthermore can resist current overloads without loosing their magnetic characteristic.
However, to generate sufficient torques with such low flux density magnets, these permanent magnets must have a larger volume. Since the permanent magnets are inserted into a rotor core from the outer rotor diameter, these magnets must be small in a direction perpendicular to the median plane (circumferential). Thus, in order to realize the necessary volume, these magnets at the same time must be large in a radial direction that results in large rotor diameters, thereby increasing the overall size of the traction drive. This is disadvantageous in particular with elevators with a small or no machinery room.
As a result, providing said permanent magnets within a rotor core, that is assembled on a shaft for torque transmission, presents its normal difficult in manufacturing, limits the size and thereby the magnetic flux of the such-integrated permanent magnets and increases the rotor diameter due to the separated rotor core.